1. Field of the Invention
The present invention relates to superplastic forming processes and more particularly to an apparatus and method for superplastic forming using an autoclave.
2. Description of the Related Art
It is well known that under the proper conditions certain materials exhibit superplasticity. Superplasticity is the capability of a material to develop unusually high tensile elongations with reduced tendency toward necking, a capability exhibited by only a few metals and alloys and within a limited temperature and strain rate range. Titanium, aluminum, and alloys therefrom are generally the materials chosen when superplastic forming techniques are implemented because these materials exhibit superplastic characteristics equal to or greater than those of any other metals. With suitable titanium alloys, overall increase in surface area of up to 300% are possible.
The advantages of superplastic forming are numerous. For example, very complex shapes and deep drawn parts can be readily formed. Low deformation stresses are required to form the metal at the superplastic temperature range, thereby permitting forming of parts under low pressures which minimize tool deformation and wear. This process allows the use of inexpensive tooling materials, and eliminates creep in the tool. Single male or female tools can be used. No spring-back occurs or Bauschinger effect develops. Multiple parts of different geometry can be made during a single operation and very small radii can be Formed. An example of a method used for the superplastic forming of metals is disclosed in U.S. Pat. No. 3,934,441, issued to Hamilton et al, entitled "Controlled Environment Superplastic Forming of Metals," assigned to the present assignee.
Currently, most superplastic forming techniques utilize an assembly having an upper press platen and a lower platen. Adjacent the lower surface of the upper press platen is a lid. A die is located adjacent the upper surface of the lower platen. The metal sheet to be superplastically formed is maintained between the lid and the die. The lid contains an inlet for providing pressurized gas to the upper side of the metal sheet at the necessary pressure for superplastic forming. The die contains an access for supplying gas to or removing gas from the die cavity. Seals are maintained between the lid and the metal sheet and between the metal sheet and the die. Typically, a hydraulic ram is used to properly seat the seals. Frequently, a desired pressure, i.e. back pressure that may be several times higher than the forming pressure, is maintained in the die cavity to minimize grain boundary cavitation. Application of such back pressure necessitates that pressures for superplastic formation be concomitantly increased. The resulting high pressures severely jeopardize the integrity of the seals.
U.S. Pat. No. 4,288,021, issued to W. Leodolter, entitled "Tooling for Superplastic Forming Diffusion Bonding Processes" discloses a tooling for use in an autoclave or a hot press for the superplastic forming and diffusion bonding of metals. The tooling includes upper and lower portions with a labyrinth seal therebetween having inert counterflowing gas introduced thereto to prevent contamination from flowing across the seal to attack the metal being processed. The labyrinth seal consists of sealing tubes defining two passages to be vented to remove gas and thereby prevent part contamination by the carbon dioxide gas used to pressurize the autoclave. The autoclave carbon dioxide pressure is used to effect potentially leaky tubing seals and the titanium spacing ring seal. No provision is made in the '021 device for the use of back pressure. Thus, there are potential grain boundary cavitation problems. Further, if the use of back pressure were attempted, the seals would have to act against the back pressure plus the forming pressure inasmuch as at least one of spaces of the labyrinth seals is vented to the atmosphere.